When a conventional vehicle such as a passenger automobile or truck turns, each of the four wheels on the vehicle can rotate at different speeds. The wheels on the side of the vehicle facing into the turn, for example, typically spin at a slower rate than those on the outside of the turn due to the shorter distance traversed by the inside wheels. Front and rear wheels similarly traverse different distances around the radius of the turn, thereby resulting in unequal turning speeds. For non-driven wheels (e.g., the front wheels in a rear-wheel-drive automobile), the difference in speed is easily accommodated. For wheels that are driven with engine torque, however, the wheels must be mechanically allowed to spin at different rates to prevent spinning or slippage of one or more wheels.
To inhibit the potential for wheel spin, various types of differential driveline devices have been used to control the distribution of torque from the vehicle engine to the spinning shafts attached to each wheel. Each set of driven wheels in a modern vehicle typically contains at least one differential. Front wheel drive (FWD) vehicles, for example, typically include front differentials that allow the front wheels to turn at different speeds. Rear wheel drive (RWD) vehicles similarly include rear differentials that allow rear wheels to turn at different speeds. Four-wheel drive (4WD) vehicles typically include both front and rear differentials. Additionally, all wheel drive (AWD) and full-time 4WD vehicles typically include center differentials that allow the front and rear wheels to rotate at different rates during turns.
More recently, active driveline devices commonly known as electronically-controlled limited slip differentials (eLSD) have become increasingly popular. Typically, the eLSD include a slip control algorithm that provides controlled variable coupling of the engine's driving torque to two or more of the vehicle wheels through the use of an electrically actuated clutch. Under normal driving conditions, such eLSDs typically function as open differentials that evenly distribute torque to the wheel shafts. When a loss of traction is sensed at one output of the differential, however, the eLSD clutch can be activated via a feedback control loop to maintain a speed difference between the differential outputs that would result as if the wheels were rotating at their natural speeds, thereby improving the stability and comfort of the vehicle.
Generally speaking, most eLSD implementations use vehicle reference speeds to calculate appropriate control values for the differential outputs. Challenges often arise, however, in that accurate vehicle reference speed measurements can be difficult to obtain in practice. eLSD computations that determine desired wheel speed based upon vehicle reference speeds, then, can occasionally produce driver discomfort or other undesired results. In particular, vehicle investigations have shown that even slight differences in the determination of target wheel speed can result in improper clutch activation during low-speed, tight-turn applications such as parking-lot maneuvers and the like.
Accordingly, it is desirable to provide methods and devices for determining target wheel velocities that are independent of vehicle reference speed, and that will enable the slip control algorithm of an eLSD to correct for longitudinal tire slip, especially in tight turn situations. In addition, it is desirable to provide eLSD corrections for front, center, and rear driveline device applications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.